I think looking back the two biggest mistakes that Ainge, the coaching staff, the management (whatever you want to call it, call it all three...) did were:
1. Playing guys like Bass, Hump, Bayless, Rondo and Green (they all should have been traded) rather than playing our young guys. This absolutely cost us a legitimate chance at drafting Parker or Wiggins.
2. Perhaps even more criminal in nature was playing guys like Bass, Prince, Thornton and Green just enough minutes to move us from the 8th to 11th pick to the ever coveted 16th pick in the 2015 draft. This likely causing us a shot at Stanley Johnson, Justice Winslow or Myles Turner.
In both 2014 and 2015 it seemed pretty clear that the above mentioned vets were clearly not in our long term plans. Why would ever play guys like Prince and Thornton, and even Bass, Green and Rondo when you know they will never be wearing a Celtics uniform again.
You don't need to showcase them - the league understands their talent level.
These were incredibly huge top down mistakes.
I agreed with a lot of your post besides this. I think you play the players that you think will win you the most games on a given night. I think it's a horrible practice to give rookies playing time on the specific reason being that they are young. Make them earn playing time in practice and when they do get opportunities.
Sitting veterans and playing rookies just to force feed development just makes players lazy and entitled, and ensures that your team is going to be bad which is never the goal.